This invention relates to a touch sensor to detect the touch of a body part and in particular, to a touch sensor to detect the touch of a human finger for use in various electronic applications and security devices.
Current touch detecting systems employ a variety of sensors, including optical, capacitance, electrical conductivity and pressure sensors. For example, a common capacitance sensor is described in Japanese Patent Publication (Kokai) 10-165382. The inventors previously developed a human body sensor which detects the presence of a person using high-frequency radiation (Patent publication (Kokai) 9-46205). In this device, illustrated in FIG. 1, a high-frequency signal is emitted by an oscillator 101. This signal is transmitted to a sensing unit 102. The presence of a human body changes the impedance of the circuit and causes the amplitude of the wave reflected by sensing unit 102 to vary along with it. The amplitude of this reflected wave is detected by a detector unit 103, and a control unit 104 determines from this level whether a person is present. In particular, FIG. 18 illustrates an exemplary circuit for the detector unit used in the configuration shown in FIG. 1.
Conventional touch sensors, however, do not have a capacity to determine whether the object touching the sensor is actually a finger. While some of the conventional inductance, electrical conductivity and high-frequency radiation sensors are not supposed to react unless they are touched by a finger, it is relatively easy to operate the sensors by intentionally touching them with an object having characteristics similar to a finger. That is, the conventional sensors are unable to properly discriminate. Detecting coils, for example, detect the presence of a human body before they are actually being touched, so they have a problem of accuracy in that they cannot determine whether the sensor was in fact touched by the human body.
The invention provides a touch sensor which is able to detect the touch of a finger with reasonable accuracy.
In one embodiment of the invention, a touch sensor includes an oscillator unit which outputs a high-frequency signal; an electrode unit which receives the high-frequency signal from the oscillator unit and which includes electrodes that are touched by the finger to be detected; a detector unit which outputs a signal which varies with the impedance of the electrode unit; a discriminator unit which determines whether the item detected is alive; and a reference unit in which a reference signal is stored prior to use to determine whether the item detected is a human body part.
In this touch sensor, the oscillator unit supplies a high-frequency signal to the electrode unit. If the item detected is a biological entity of a human body such as a finger, then the impedance of the electrode unit will change. The impedance is initially composed to the impedance on the input side of the electrode when the finger is touching the electrodes. The reflected wave will be smaller due to the change in impedance when the finger is touching the electrodes. By detecting this reflected wave and comparing it with a reference signal in the discriminator unit, it can be determined that the reflection level is lower and that the electrodes are being touched by a person.
In another embodiment of the Invention, a method of sensing a touch of a body part includes . . .